1. Field of the Invention
The present invention relates to a wide-angle zoom lens. In particular, the wide-angle zoom lens is suitable for use with a shutter camera with no restriction in back focus.
2. Background of Related Art
Lens shutter type cameras that are equipped with zoom lenses are very common. In recent years, the number of cameras equipped with high zooming ratio zoom lenses with a zooming ratio exceeding two and a wider range of angles have increased. Lens shutter type camera bodies have become more compact and more light weight. Generally, a zoom lens is retracted in the body as the distance between each of the lens groups is minimized during each zooming, in a lens shutter type camera with a zoom lens. Consequently, minimizing the thicknesses of the lens groups and lens diameter of each lens element in a zoom lens becomes important in making a camera body more compact.
Additionally, in lens shutter type cameras equipped with a zoom lens without restrictions in back focus, the back focus can be shortened at the maximum wide-angle state by placing a negative lens group closest to the image plane of the lens system. Furthermore, attempts have been made to range to more wide angles in lens shutter type cameras with a zoom lens by causing the height of the off-axis rays passing through a negative lens group to be separated from the optical axis. Thus, correcting fluctuations in coma aberrations that accompany changes in the field angle is easier. Further, by making the back focus larger during zooming from a maximum wide-angle state to a maximum telephoto state, the change in the height of off-axis rays and off-axis aberrations fluctuations are suppressed during zooming.
A negative-positive-negative type zoom lens is a conventional zoom type lens with a high zoom ratio and with a high range for the wide angle have high zooming ratio zoom lenses with wide angles. For example, these lenses are known in Japanese Laid-Open Patent Publications Hei 2-238,416 and Sho 54-781,149. These negative-positive-negative three group zoom lenses comprise three movable lens groups. These lens groups are, in order from the object side, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having negative refractive power. Furthermore, during zooming from the maximum wide-angle state to the maximum telephoto state, each lens group moves so the distance between the first lens group and the second lens group decreases, and the distance between the second lens group and the third lens group decreases.
However, with the conventional negative-positive-negative type zoom lens, the refractive power of the first lens group is weak, which causes separation of the height of an off-axis ray from the optical axis. Therefore, the off-axis aberrations and the on-axis aberrations are independently corrected. However, if a wide-angle is desired, the off-axis ray may be further separated from the optical axis. Thus, it is difficult to properly minimize lens diameter.
Moreover, in a zoom lens where negative lens groups are closest to the image side, a sufficient back focus can not be obtained at a maximum wide-angle state to achieve a wide-angle lens state. Therefore, a shadow of any debris on the rearmost lens surface can be each recorded on the film's surface.
In the zoom lens in Japanese Laid-Open Patent Publication Hei 2-238416, a field angle wider than 70.degree. can be included at a maximum wide-angle state. However, sufficient back focus could not be obtained at the maximum wide-angle state. Thus, a shadow of any debris on the rearmost lens surface can be easily recorded on the film's surface.
Several methods for focusing at close range by moving only one of the zoom lens groups of the lens system are also known. When focusing at close distances from a far distance, the following three methods of focusing are known:
(A) the FF (front focus) method; PA1 (B) the IF (inner focus) method; and PA1 (C) the RF (rear focus) method.
For these methods, focusing lens unit, which moves along the optical axis for focusing at close distances, must have a small lens diameter and a small amount of movement. If these two conditions are satisfied, the lens structure is simplified, and the lens system can be made smaller.
When focusing at a close distance with known negative-positive-negative three group zoom lens, the FF method (A) and the RF method (C) can be used. However, in negative-positive-negative three group zoom lenses, the diameters of first and third lens groups would be large. Thus, the FF method (A) and RF method (C) are not suitable for compactness.
Moreover, in zooming from a maximum wide-angle state to a maximum telephoto state, the second lens group could not be used as a focusing lens group by the IF method (B), because the lateral magnification of the second lens group changes from the first range (from -1 to 0) to the second range (less than -1), and the direction of movement of the second lens group is opposite that which is needed between the first range and the second range. This is not suitable for the known methods.
Moreover, with known negative-positive-negative three group zoom lens such as disclosed in Japanese Laid-Open Patent Publication Sho 54-78149, the back focus may be restricted. Consequently, the vertical height difference between the on and off-axis rays, which pass through the third lens group at a maximum wide-angle state, is small. The change in height of the off-axis rays is small when zooming from the maximum wide-angle state to the maximum telephoto state. Therefore, the aberration correction burden on the third lens group will be small, and it will be difficult to increase the zooming ratio.
Further, as auto focus cameras have become more common, the various methods have been used for focusing at close distances. For an optimum zoom lens in an auto focus camera, the focusing lens group, which moves during focusing at close distances, must be small. Also, the work displacement (displacement.times.mass) during focusing at close distances must be small to improve auto-focus systems with high speed focusing and a simple structure.
In a negative-positive-negative type zoom lens, as disclosed in U.S. Pat. No. 5,268,792, the lens unit of the second lens group, which is arranged closest to the object side, is used as the focusing lens group and may be formed from a single lens. Consequently, its refractive power could not be strengthened to adequately correct fluctuations in various aberrations created during focusing. It is impossible to shorten the total length of the lens system at the maximum telephoto state, because the principal point of the second lens group is positioned on the object side.